1. Field of Invention
The invention relates to a method of blanket re-processing multilayer circuit boards with defective external copper conductors.
2. Description of Prior Art
In the course of the miniaturization of electronic circuits, it is common to fabricate circuit boards having several layers of insulating substrate and conductive planes sandwiched together. In practice, a copper foil is laminated to an insulating substrate and a circuit pattern is subsequently defined using known photolithographic techniques. A second insulating plane is then laminated over the copper conductive plane, followed by a new copper plane for the next circuit layer. This process can continue for several different layers on either or both sides of the original circuit plane. It is not uncommon to encounter defective conductors in an external plane of the multilayer circuit board after several conductive planes have already been fabricated.
When one or two conductors are defective, it is possible to repair them using known bridging or repair techniques. However, when there are several defects in the external conductive plane or when the copper conductors are very closely spaced, these repair methods are no longer economically feasible. The only alternatives available are either to scrap the multilayer circuit board, an expensive and inefficient alternative, or re-work the external defective plane by etching off the external copper conductors and replacing them. Unfortunately, no method has been known which permits the additive metal plating of external conductors on a partially fabricated multilayer circuit board.
To successfully employ the additive metal plating process, an activating layer, e.g., a thin copper foil, must be applied on the insulating substrate as a seed on which conductors will then grow. The application of this activating or seed layer, however, has presented difficult problems. Experience has shown that it is impossible to relaminate a copper foil on an etched cured epoxide resin substrate. This would only be possible if a pre-crosslinked epoxide resin substrate were used, but even then, if through holes were present, the multilayer circuit board would have to be newly drilled, and the drills would have to hit the old holes precisely to avoid their fracturing. In the past, this has not been possible or practical so the multilayer circuit boards have been scrapped, resulting in a significant loss of time, resources and money.
Therefore, it is the object of this invention to provide an improved method according to which multilayer circuit boards with defective external copper conductors can be blanket re-processed to allow the salvage of valuable multilayer circuit boards.
It is also an object of this invention to provide a method according to which a copper activating or seed layer can be applied to a cured epoxide resin insulating substrate without the problem of delamination.
It is still another object of this invention to provide a method according to which copper plated through holes can be etched at a rate decreasing towards the center of the wall of the holes, thereby protecting the internal conductor planes.